Needs have arisen for the printing of large banners, flags and signs in quantities that are not economical for many conventional printing processes. Proposals have been made to print such products from electronic source files that can be processed directly on the printing press or printing system, rather than through steps such as film image-setting and plate-making. One such process is ink jet printing. These processes have been attempted on surfaces such as vinyl, but printing with success onto textile surfaces has been even more limited. Such processes have been slow and lack reliability. The clogging of print heads in ink jet printing has been too frequent for use in wide width and large area substrates, and the processes used have not produced acceptable printing on textile materials.
The printing of substrates that are more than several feet, or a meter, wide, referred to as the special category of “wide width” printing, into which category the printing of signs and banners, office partitions, mattress ticking and most other quiltable materials would fall, is beyond many of the limitations of conventional printing methods. A number of technical problems exist that have deterred the development of the printing of wide fabrics such as mattress covers, upholstery, automobile seat cover fabrics, office partitions and other wide width substrates.
Wide width products are frequently printed in relatively small quantities. Traditional printing typically involves the creation of a plate, a mat, a screen, or some other permanent or at least tangible, physical image from which ink is transferred to the object being printed. Such images contribute a relatively high set up cost that is only economical where the number of identical copies of the product is large. At the other extreme, office printers, for example, print a single copy or a small number of copies of a given document or other item, and are currently of the type that uses no permanent, physical image transfer element, but which rather prints from a software or program controlled electronic image, which can be changed from product to product. Such printing is sometimes referred to as direct digital printing, although the process need not necessarily be literally “digital” in the sense of a set of stored discrete numerical values. Ink jet printers are a common type of such direct digital printers in use today.
Ink jet printers print by projecting drops of ink on demand onto a substrate from one or more nozzles on one or more print heads. Office printers and other narrow width ink jet printers usually dispense water based or other solvent based inks onto the substrate by heating the ink and exploding bubbles of the ink out of the nozzles. These printers are often called bubble jet printers. The ink from such printers dries by evaporation of a solvent. Sometimes additional heat is used to evaporate the solvent and dry the ink. Printing onto wide width substrates with bubble type ink jet printers, or ink jet printers that use high temperature techniques to propel the ink, suffer from limited printhead life or high mean time between failures that require downtime and servicing. The heat used to expel the ink and to cause the evaporation of the solvents, evaporation that occurs during printhead downtime, and the thermal cycling of the heads, causes these print heads to clog or otherwise fail after as little as 20 milliliters of ink is dispensed. Office printers are, for example, often designed so that the print head is replaced every time a reservoir of ink is replenished. For this reason, for larger scale ink jet printing processes, such as wide width printing of films used for outdoor advertising, signage and architectural applications, print heads that use mechanical ink propulsion techniques are more common. Such mechanical print heads include piezo or piezo-crystal print heads, which convert electrical energy into intra-crystal vibrations that cause drops of ink to be ejected from print head nozzles.
Piezo print heads are particularly useful for applying inks that dry by polymerization which can be brought about after the ink leaves the print head and is deposited onto the substrate, usually by exposure to some form of energy medium such as electromagnetic or particle radiation. Inks have been formulated for ink jet printing that can be polymerized by exposure to a radiation curing source such as a focused beam of ultra violet light (UV) or high energy beams of electrons (EB). The inks generally incorporate stabilizers which prevent premature curing due to low levels of light exposure. Therefore, the inks usually require exposure to some threshold level of energy to initiate a polymerization reaction. Unless exposed to such threshold energy levels, such inks do not polymerize and remain stable, with a low tendency to dry in the nozzles or elsewhere unless cured by adequate exposure to the energy medium.
Solvent based inks are primarily cured by evaporation of the solvents. Some solvent based inks can be cured only by air drying, while others require the application of heat to enhance the evaporation of the solvent. In some cases, heat will facilitate a chemical change or polymerization of the ink along with an evaporation of a solvent. Polymerizable inks include monomers and oligomers that polymerize, and other additives. UV curable inks polymerize when exposed to UV light at or above the threshold energy level. These UV curable ink formulations include photo-initiators which absorb light and thereby produce free radicals or cations which induce cross-linking between the unsaturation sites of the monomers, oligomers and polymers, as well as other additive components. Electron beam-cured inks do not require photo-inhibitors because the electrons are able to directly initiate cross-linking.
Heat or air curable inks that are organic solvent based or water based inks often do not have as high a color intensity as UV curable or other polymerizable inks because the pigments or dyes that produce the color are somewhat diluted by the solvent. Furthermore, organic solvents can produce an occupational hazard, requiring costly measures be taken to minimize contact of the evaporating solvents by workers and to minimize other risks such as the risks of fire. Solvent based inks, whether applied with heat or not, tend to dry out and eventually clog ink jet nozzles. In addition, solvent based inks set by forming a chemical bond with the substrate, and accordingly, their formulation is substrate material dependent. As a result, the selection of solvent based ink varies from fabric to fabric. Specific ink compositions are paired with specific fabric compositions to improve the fastness of the ink to the fabric, which results from chemical or electrostatic bonds formed between the ink and the fabric. Where the selected ink composition does not react or otherwise has an affinity with the surface of the particular fabric, the ink merely maintains a physical contact with the fabric surface and typically is easily removed by water, another solvent or abrasion. With UV and other radiant beam-curable inks such as electron beam-cured inks, the bonding between the ink and fabric is primarily mechanical and not limited to specific combinations of ink and fabric.
Polymerizable inks, particularly those cured upon exposure to a radiation or energy medium, are difficult to cure on three dimensional substrates such as the surface of a textile. While UV curable inks are capable of providing higher color intensity and do not present the hazards that many solvent based inks present and can avoid nozzle clogging, printing with UV curable ink onto textile fabric presents other problems that have not been solved in the prior art. To cure UV ink, for example, it must be possible to precisely focus a UV curing light onto the ink. UV ink, when jetted onto fabric, particularly onto highly textured fabric, is distributed at various depths over the texture of the fabric surface. Furthermore, the ink tends to soak into or wick into the fabric. As a result, the ink is present at various depths on the fabric, so that some of the ink at depths above or below the focal plane of the UV curing light evade the light needed to cause a total cure of the ink. In order to cure, UV ink must be exposed to UV light at an energy level above a curing threshold. However, increasing the intensity of the curing light beyond certain levels in order to enhance cure of the ink can burn, scorch or otherwise have destructive effects on the deposited ink or the fabric. Furthermore, ink jet printing can be carried out with different ink color dots applied in a side-by-side pattern or in a dot-on-dot (or drop-on-drop) pattern, with the dot-on-dot method being capable of producing a higher color density, but the higher density dot-on-dot pattern is even more difficult to cure when the cure is by UV light.
In addition, UV ink can be applied quickly to reduce wicking and UV ink can be developed to allow minimized wicking. Some wicking, however, can help to remove artifacts. Further, many inks developed to eliminate wicking leave a stiff paint-like layer on the surface of the fabric, giving the fabric a stiff feel or “bad hand”. Therefore, to reduce the UV curing problem by eliminating wicking is not always desirable.
UV curing of jetted ink on fabric has been plagued by a limited cure depth that is determined by the depth of field of the focused curing UV light. When UV curable ink is jetted onto fabric, UV light may be ineffective to cure a sufficient portion of the ink. A large uncured portion of the deposited ink can cause movement of the ink or the loss of the ink over time, resulting in deterioration of the printed images. Even if a sufficient portion of the ink is cured to avoid visibly detectable effects, uncured ink at some level has the possibility of producing symptoms in some persons who contact the printed fabric. The amount of uncured monomers or ink components that can cause problems by inhalation or direct skin contact has not been officially determined, but standards exist for determining limits for components of packaging material ingested with food. For example, if more than approximately 100 parts per million (PPM) of ink from packaging material is present in food, some persons who are sensitive to the uncured monomers may suffer reactions and others may develop sensitivities to the material. Such criteria assumes that 1 square inch of packaging material makes contact with ten grams of food. Thus, to interpret this criteria, it is assumed that each PPM of ink component in packaged food is equivalent to 15.5 milligrams of ink component migrating out of each square meter of packaging material into the food. While this does not provide an exact measure of the amount of uncured ink components that might be harmful to humans, it suggests that approximately 10% of uncured ink components on items of clothing, mattress covers or other fabrics with which persons may be in contact for extended periods of time, may be unacceptable.
For the reasons stated above, UV curable inks have not been successfully used to print onto fabric where a high degree of cure is required. Heat curable or other solvent based inks that dry by evaporation can be cured on fabric. As a result, the ink jet printing of solvent based inks and heat curable or air dryable solvent based ink has been the primary process used to print on fabric. Accordingly, the advantages of UV or other radiation curable ink jet printing have not been available for printing onto fabric.
UV inks, other polymerizable inks and other stable inks are typically those that reside on the surface of the substrate. The color components of the inks are in the form of pigments suspended in a polymer or other curable matrix. When the printed substrate is washed or exposed to weather or wear, the ink coating usually fades or otherwise degrades. Inks containing dyes, on the other hand, provide color fastness because the dye dissipates into and becomes chemically or mechanically bonded to the fibers of the substrate. Such dye-based inks are particularly useful in printing on polyester substrates, where sublimation dyes effectively bond to the polyester fibers. But because such inks employing dyes as the color component have traditionally required a solvent to suspend and carry the dye to the substrate, dye-based inks have resulted in “drop-spread”, wicking of the ink, or blurring of the images that are being printed. As a result, the need to reduce this drop-spread with dye-based inks has necessitated the use of transfer processes rather than direct digital printing.
Furthermore, in the ink jet printing of textiles, specifically those made of porous materials or open weave fabrics, the jetted ink passes through holes in the substrate and deposits onto the substrate support. Traditionally, an absorbent blotter-like layer is placed under the substrate to collect the excess ink. The handling and disposal of the ink carrying layer is messy and inconvenient.
There exists a need in printing of patterns onto mattress ticking and mattress cover quilts, as well as onto other types of fabrics, for a process to bring about an effective cure of ink compositions containing UV curable inks and to render practical the printing with UV curable inks onto fabric for clog free ink-jet printing with stable inks that are completely curable, result in color fast images, with a minimum of drop spread. Additionally, a better way is needed for handling excess ink that passes through porous textiles in an ink-jet printing process.